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Traffic and Process Capability (Ppk) Relate?
As a teacher and a mentor, I am constantly looking for new ways to make complex subjects less complex using games, simulations stories, analogies, etc. My colleagues and I have found that the topic of Process Capability (PPK) vs CPK and how to interpret the Long Term and Short Term capability indices tend to make our student’s “eyes glaze over”.
What we came up with is a spin on other examples that I found on the web. We use an analogy of a car in a lane of a highway. The lane is bordered on both sides by concrete barriers. Cars are 60+ miles per hour in this lane.
Before we go into the analogy, we must first define what the CAR and the LANE represent.
- The Width of the CAR is the Voice of the Process (VOP).
- The Width of the Car (or VOP) is always equal to six standard deviations wide.
- VOP is a dynamic metric because as we add more data to the data set the standard deviation changes
- The Width of the Lane (or VOC).
Now that we have a better understanding of how the analogy of the CAR and the LANE relate to VOP and VOC, we can use the CAR and the LANE example to explain the Process Capability Indices Ppk.
We only explain the Capability Indices Ppk because it is considered “Real Capability” and will give you a truer representation of your process capability to meet the requirements set by the customer.
Scenario:
You are in a city that has a road with one of two lanes under construction.
The are concrete barriers on both sides of the single lane.
Cars are going 60 MPH or more in this single-lane
The Capability Indices “Pp” Compares the VOP (Car) to the VOC (Lane)
The Calculation for Pp = VOC / VOP
In this example Pp = 12/12 = 1
The Calculation for VOC = USL – LSL
The Calculation for VOP = 6 x StDev
“Pp” does not care whether the car is in the lane. It just compares the size of the lane to the size of the car
The Capability Indices “Ppk” not only Compares the VOP to the VOC but also tells you where the car is in the lane.
Ppk is the Minimum of two Calculations Ppu (Pp Upper) and Ppl (Pp Lower)
The Minimum of the two calculations tells us what side of the Center of Lane the is the Center of the Car
The Calculation for Ppk = Min (Ppu, Ppl)
- Ppu = (USL – Mean) / (3 x StDev)
- Ppl = (Mean – LSL) / (3 x StDev )
Ppk = Min of Ppu and Ppl
Can Ppk be less than 1?
The Calculation for Ppk = Min (Ppu, Ppl)
- Ppu = (USL – Mean) / (3 x StDev) (in this example: (12 – 3) / 5 = 1.80)
- Ppl = (Mean – LSL) / (3 x StDev) (in this example: (3 – 0) / 5 = 0.6)
Ppk = Ppl (in this example: 0.6)
If the side of the car exceeds the lane then the Ppk is less than 1
If the center of the car has not exceeded the lane but the side of the car has then the Ppk is less than 1 and greater than 0
Can Ppk be 0 (Zero)?
The Calculation for Ppk = Min (Ppu, Ppl)
- Ppu = (USL – Mean) / (3 x StDev) (in this example: (12 – 0 )/ 5 = 2.40)
- Ppl = (Mean – LSL) / (3 x StDev) (in this example: (0 – 0) / 5 = 0.0)
Ppk = Ppl (in this example: 0.0)
If the middle of the car equals the lane then the Ppk is 0 (Zero)
Can Ppk be less than 0?
The Calculation for Ppk = Min (Ppu, Ppl)
- Ppu = (USL – Mean) / (3 x StDev) (in this example: (12 – 13) / 5 = -0.2)
- Ppl = Mean – LSL / 3 x StDev (in this example: (13 – 0 )/ 5 = 2.6)
Ppk = Ppu (in this example: -0.2)
If the middle of the car exceeds the lane then the Ppk is less than 0 (zero) or negative
What if the Car is in the Middle of the Lane?
The Calculation for Ppk = Min (Ppu, Ppl)
- Ppu = (USL – Mean) / (3 x StDev) (in this example: (12 – 6) / 5 = 1.2)
- Ppl = (Mean – LSL) / (3 x StDev) (in this example: (6 – 0) / 5 = 1.2)
Ppk = Ppu (in this example: 1.2)
The Calculation for Pp = VOC / VOP (In this example Pp = 12/10 = 1.2)
If the middle of my car equals the middle of the lane, then Pp, Ppu, Ppl, and Ppk are the same
Feel free to use this analogy to teach others the concept of the Process Capability Indices Ppk. If this article has helped you or your colleagues to better understand these concepts please share with us in the comments below.
PPK vs. CPK: Key Differences
It can be difficult to understand these concepts if you don’t work with them regularly. There are many resources available in print and online to help you calculate Cpk vs Ppk. This article is meant to explain why it’s important to calculate the Cpk and Ppk differently.
Cpk (process capability) vs Ppk (process performance) are both shorthand terms for the same thing.
The process capability (Cpk) is important as it shows whether a particular process can potentially meet a specification. It is a forecast of a process’ ability to meet a specification.
Process performance (Ppk) is important as it shows how a process has performed over time. It is a measure of how well the process is performed.
The key point is that Cpk represents the potential for a process (short-term) to meet a specification, while Ppk represents how the process actually performed (long-term).
Cpk can be used to represent a subgroup, while Ppk represents the entire process, vs Cpk is used to determine if a process can meet the specification under ideal conditions. Ppk is more realistic, as it does not remove real data. Cpk can inadvertently alter the data to make it appear that a process has a high level of capability without addressing its actual performance.
